Which is a stronger reducing agent , `SbH_(3)` of `BiH_(3)` , and why ?

Explanation

As we move down the group 15 elements from Sb to Bi, the atomic size increases and hence the strength of the bond it makes with hydrogen decreases, i.e., Bi-H bond is weaker than Sb-H bond. In other words, `BiH_(3)` is thermally less stable than `SbH_(3)`. Therefore, on heating, `BiH_(3)` has a greater tendency to liberate hydrogen than `SbH_(3)` and hence `BiH_(3)` is a stronger reducing agent than `SbH_(3)`.